1. Field of the Invention
The present invention relates to a gas sensor with at least one gas-sensitive, electrically conductive layer, having a surface region which can be brought into contact with a target gas and in which the work function depends on the concentration of the target gas in contact therewith, and with at least one electric potential sensor capacitively coupled to the surface region via an air gap.
2. Description of the Background Art
A gas sensor for measuring the hydrogen gas concentration is disclosed in DE 43 33 875 C2. The gas sensor has a silicon substrate, in which as a potential sensor a field-effect transistor with a drain, a source, and a channel region located between these is integrated. An electrical insulation layer is arranged on the channel region and a gate electrode thereupon. To the side of the gate electrode, a sensor electrode is provided, which is connected integrally to the gate electrode to form a suspended gate. On its lower side facing the substrate, the sensor electrode is coated with a gas-sensitive layer, which is capacitively coupled to the source via the air gap. A surface region, facing the substrate, of the gas-sensitive layer can be brought into contact with the hydrogen gas, which during the contacting of the surface region is adsorbed on said region. With a change in the hydrogen gas concentration, the work function changes in the surface region of the gas-sensitive layer. Because the sensor electrode is capacitively coupled to the surface region and connected to the gate electrode, the electric potential at the gate electrode also changes thereby. The current flow between the drain and source is controlled depending on the change in potential.
In normal indoor air, a thin layer of atmospheric oxygen is adsorbed dissociatively on the surface of the gas-sensitive layer, i.e., as oxygen atoms, not as oxygen molecules as they occur in air. When the target gas enters the vicinity of the gas-sensitive layer, an adsorption of the target gas on the surface occurs first, whereby the target gas partially displaces the atmospheric oxygen adsorbed on the surface and occupies its adsorption sites. Both effects, the adsorption of the target gas and reduction of the oxygen occupancy, contribute additively to the change in the surface work function. At the same time, however, a reaction between hydrogen and oxygen, during which water is formed, occurs on the surface, promoted by the catalytic action of the gas-sensitive layer. As a result, at low temperatures below about 60° C., only the hydrogen coverage on the surface is reduced gradually. This hydrogen consumption is compensated by continuous new adsorption of hydrogen from the gas phase, so that a stable measuring signal is assured. At higher temperatures above about 60° C., the reaction proceeds so rapidly that the hydrogen adsorption can no longer fully compensate for the hydrogen consumption by the surface reaction and, additionally, also the hydrogen concentration in the immediate vicinity of the gas-sensitive layer is reduced. The oxygen coverage of the surface can again increase as a result. All three effects shift the work function in the opposite direction. This reaction can occur within hours or also within seconds, depending on the temperature of the gas-sensitive layer, so that the measuring signal can be greatly disrupted.